1. Field of the Invention
The present invention relates to an optical device of a periodic inverted domain construction suitable for application, for example, to a second harmonic generating device (hereinafter referred to as a "SHG device").
2. Description of Related Art
Recently, techniques have been proposed so as to improve the optical output characteristics of optical devices including SHG devices by forming a periodic inverted domain structure in the surface of a ferroelectric crystal. When a SHG device, for instance, is exposed to light having a frequency of .omega., the SHG device generates a second harmonic having a frequency of 2.omega.. Thus, the SHG device expands the wavelength range of light having a single wavelength. This effect of the SHG device expands the field of application of lasers and optimizes the use of lasers in various technical fields. For example, the reduction of the wavelength of a laser beam enables the increase in the recording density for optical recording and reproducing using a laser beam and in magneto-optic recording and reproducing using a laser beam.
There have been proposed a bulk SHG device employing NbO.sub.3, and a SHG device of a waveguide type which uses a comparatively large nonlinear optical constant for phase matching, such as a Cerenkov radiation SHG device employing a linear waveguide formed on a single crystal substrate formed of a nonlinear optical material, such as a ferro-electric single crystal of LiNbO.sub.3 (LN), and which is capable of emitting a second harmonic, such as green light or blue light, through the substrate in a radiation mode upon the reception of a fundamental wave, such as a near-infrared radiation.
However, the known bulk SHG device has a comparatively low SH-conversion efficiency owing to its intrinsic characteristics and it is unable to employ inexpensive, high-quality LN. The Cerenkov radiation SHG device emits a second harmonic beam into the substrate, and the second harmonic beam emitted by the Cerenkov radiation SHG device forms a spot having an irregular shape, such as a crescent spot, which produces problems in the practical application of the Cerenkov radiation SHG device.
To enable a SHG device to operate at a high conversion efficiency, the respective phase propagation velocities of the fundamental wave and the second harmonic must be the same. The respective phase propagation velocities of the fundamental wave and the second harmonic can be made to coincide with each other by a method proposed in the publication J. A. Armstrong, N. Bloombergen, et al., Phys. Rev., 127, 1918 (1962), in which the "+" and "-" of nonlinear optical constants are periodically arranged. Such an arrangement of the nonlinear optical constant can be achieved by periodically inverting the orientation of the crystallographic axis. The inversion of the orientation of the crystallographic axis can be achieved by a method employing a laminated structure of slices of a crystal such as described in Okada, Takizawa and leiri, NHK Gijutsu Kenkyu 29(1), 24 (1977) or by methods employing a periodic domain inversion structure formed by controlling the polarity of the current supplied in forming a crystal by a pulling method as described in D Feng, N. B. Ming, J. F. Hong, et al., Appl. Phys. Lett. 6,228 (1965) A. Foist, P. Koidl, Appl. Phys. Lett. 47, 1125 (1985). These methods desire to form a periodic structure entirely in a crystal. However, the foregoing methods require large apparatus and there is difficulty in controlling the formation of domains.
There has been proposed a method of diffusing Ti into the surface of a crystal to form a periodic domain inversion structure in the surface of the crystal as described in H. Ito, E. Cho, F. Inaba, 49th Oyo Butsuri Gakkai Koen Yoko-shu, 919 (1988). However, the refractive index of the inverted domain formed by this method changes and it is possible that the domain inversion structure emits a plurality of light beams and, in some cases, the fundamental wave leaks.
The applicant of the present patent application proposed a domain method to control the domain of nonlinear optical ferroelectric crystals in Japanese Patent Application No. 1-184362. This method disposes electrodes directly opposite to or indirectly through insulators opposite on the opposite major surfaces of a single-domain ferroelectric crystal, and applies a DC voltage across the electrodes to form local inverted domains to obtain a periodic inverted domain structure. However, as shown in FIGS. 19A and 19B, a periodic inverted domain structure formed by this method has a ratio of t/W=1 or below, where w is the width of an inverted domain region and t is the thickness of the inverted domain region. Therefore, a minute periodic inverted domain structure is formed and the value of the thickness t becomes smaller than the thickness of an optical waveguide. That is, if the width w is about 1.5 .mu.m so as to form inverted domain regions with a small pitch, the thickness t becomes as small as about 0.5 .mu.m. Therefore, if the thickness of the optical waveguide is about 1.0 .mu.m, it is impossible to make the respective phase propagation velocities coincide with each other by periodically arranging the "+" and "-" of the nonlinear optical constant because a periodic inverted domain structure cannot be properly formed in the optical waveguide portion and in the evanescent region, which is one of the causes that prevent the improvement of the efficiency of a SHG device.
A further method of forming a periodic inverted domain structure of a desired pattern by irradiating a nonlinear optical material with an electron beam is proposed in the article of R. W. Keys, A. Loni, B. J. Luff, P. D. Townsend et al., Electronics Letters 1st February 1990 Vol. 26, No. 3. As shown in FIG. 20, this method forms 50 nm thick NiCr layers 62 over the c-faces 61C of a LN substrate 61, i.e., a substrate of nonlinear optical material, and forms 400 nm thick Au layers 63 over the NiCr layers 62, and patterns the Au layer 63 in a predetermined pattern, and irradiates the patterned Au layer 63 with an electron beam. The substrate 61 is heated to about 580.degree. C. and the substrate 61 is irradiated with an electron beam with a total dosage of 10.sup.17 per 9 mm.sup.2, i.e. 10.sup.16 /mm.sup.2, by applying an electric field of 10 V/cm in a direction along its c-axis. However, this method has a disadvantage in that it is possible that the surface of the nonlinear optical material is soiled by the heat treatment at a high temperature and during the application of voltage while the substrate is heated at a high temperature after patterning the electrode layer. The outward diffusion of oxygen molecules from the LN substrate 61, like the outward diffusion of Li, so as to form inverted domain regions may possibly cause the refractive index to vary due to variations of the composition.